Optical coherence tomography, usually called OCT, is an imaging technique that uses infrared light to produce detailed cross-sectional pictures of tissue, most commonly the layers of the retina in the back of your eye. It works somewhat like ultrasound, except it bounces light instead of sound waves and achieves far finer resolution because of it. Since entering clinical practice in the 1990s, OCT has become one of the most widely performed diagnostic imaging procedures in medicine, and its uses now extend well beyond eye care into cardiology, dermatology, gastroenterology, and even industrial manufacturing.
How OCT Produces an Image
At its core, OCT sends a beam of low-power light into tissue and measures the echoes that bounce back from structures at different depths. Because light travels too fast to time its round trip directly, the system uses a technique called interferometry: it splits the light beam in two, sends one half into the tissue and the other to a reference mirror, then recombines them. When the two beams rejoin, their interference pattern reveals precisely how deep each reflection came from and how strong it was. The result is a cross-sectional slice showing tissue layers, much like a tiny biopsy you can see on a screen without ever cutting the skin.
OCT’s resolution sits in a sweet spot that few other imaging tools occupy. It can distinguish structures roughly 1 to 15 micrometers apart, depending on the system, which is far sharper than ultrasound or MRI can manage at similar depths. Its depth resolution and its sideways (transverse) resolution are independent of each other, which gives engineers flexibility to optimize each one separately for different clinical tasks. The trade-off is penetration: OCT typically images only about one to three millimeters into tissue, because light scatters and absorbs as it travels deeper. That shallow window is more than enough for the retina, the inner lining of an artery, or the top layers of skin, but it rules out imaging deep organs the way CT or MRI can.
The Speed Revolution That Made It Practical
Early OCT systems acquired images slowly, one depth point at a time, which limited their usefulness for anything beyond static snapshots. A pivotal shift came with the move to Fourier-domain OCT, which captures an entire depth profile in a single measurement rather than scanning point by point. This approach delivered a sensitivity improvement of more than two orders of magnitude, and that sensitivity gain could be traded directly for speed without sacrificing image quality. Modern swept-source OCT systems image fast enough to capture three-dimensional volumes of the retina in a fraction of a second, which is why a routine eye exam now includes OCT without adding more than a minute or two to your visit.
Eye Care Is Where OCT Lives
If you have ever been to an ophthalmologist or optometrist for anything beyond a basic refraction, you have probably had an OCT scan. The technology is so embedded in eye care that it is hard to overstate its role. OCT lets clinicians see the individual layers of the retina, measure their thickness down to a few micrometers, and track changes over time with a precision that was unimaginable before its arrival.
Retinal Disease and Macular Edema
One of OCT’s earliest and most impactful uses has been in managing conditions that cause fluid to accumulate in or under the retina, such as diabetic macular edema and age-related macular degeneration. The ability to identify, localize, and quantify fluid collections made OCT indispensable for deciding when to treat with anti-VEGF injections and for monitoring whether those injections are working. Before OCT, clinicians relied heavily on fluorescein angiography and clinical examination, both of which are less sensitive to small changes in retinal thickness. OCT measurements of retinal tissue and optical density in the central part of the macula can even predict how well a patient will respond to anti-VEGF treatment, giving doctors a way to set more realistic expectations.
From a healthcare economics perspective, this guidance role has been enormous. A study examining U.S. spending from 2008 to 2015 estimated that using OCT to personalize anti-VEGF treatment for neovascular age-related macular degeneration saved the government roughly $9 billion and patients about $2.2 billion. That $9 billion figure represented a 21-fold return on the government’s original investment in developing the technology through research grants.
Glaucoma
Glaucoma damages the optic nerve, and one of the earliest signs is thinning of the retinal nerve fiber layer, the bundle of nerve cell axons that carries visual information from the retina to the brain. OCT measures this layer directly and can detect thinning before a patient notices any vision loss. Fourier-domain OCT systems can also map the ganglion cell complex, a group of inner retinal layers whose thinning correlates with glaucoma severity. In studies comparing different OCT measurements for separating glaucoma patients from healthy individuals, the retinal nerve fiber layer thickness, ganglion cell complex parameters, and optic nerve head measurements all performed well, with no single parameter clearly outperforming the others.
The Front of the Eye
OCT is not limited to the retina. Anterior segment OCT systems image the cornea, iris, and the drainage angle where fluid exits the eye. This is particularly useful for detecting angle closure, a condition where the iris physically blocks the drainage pathway and can cause dangerous spikes in eye pressure. In a study of Asian eyes, anterior segment OCT performed in the dark identified 98% of subjects found to have angle closure on traditional gonioscopy, the standard manual examination. It also flagged angle closure in nearly 45% of subjects whose angles appeared open on gonioscopy, suggesting that OCT may catch narrowing that a clinician’s direct view misses. The specificity was lower, around 55%, meaning some truly open angles were called closed. But as a screening tool for a condition that can lead to sudden, painful vision loss, high sensitivity matters more than perfect specificity.
OCT Angiography and Dye-Free Vascular Imaging
A more recent extension called OCT angiography, or OCTA, maps blood vessels without injecting any dye. It works by acquiring rapid repeated scans of the same retinal location and detecting the motion of red blood cells. Wherever blood is flowing, the signal fluctuates between consecutive scans; wherever tissue is static, it stays the same. By filtering for those fluctuations, the system builds a three-dimensional map of the vasculature, from the large retinal vessels down to the choriocapillaris, a dense capillary bed beneath the retina that nourishes the photoreceptors.
OCTA has been a practical win for both patients and clinics. Fluorescein angiography, the older standard, requires intravenous dye injection, takes longer, and carries a small risk of allergic reactions. OCTA eliminates all of that while adding depth information that flat angiography cannot provide. It has become a go-to tool for tracking diseases like diabetic retinopathy, retinal vein occlusions, and macular degeneration, where changes in blood flow are central to what is going wrong.
OCTA does have a notable weakness: shadow artifacts. When something blocks the OCT beam, such as a vitreous floater drifting in front of the retina or the edge of a small pupil, it casts a shadow that can look like an area with no blood flow. In diseased eyes, where floaters and other obstructions are common, these shadows can be mistaken for genuine perfusion loss. Automated detection methods have been developed to flag these artifacts so they do not skew measurements of disease progression.
Inside the Arteries
Cardiologists have adopted a miniaturized version of OCT to image coronary arteries from the inside. A tiny fiber-optic probe, threaded through a catheter during an angioplasty procedure, spins and pulls back to create a detailed cross-sectional movie of the artery wall. The resolution, roughly 10 to 20 micrometers, is about ten times sharper than intravascular ultrasound, the other main tool for looking inside arteries.
This resolution matters most for identifying vulnerable plaques, the kind that can rupture and trigger a heart attack. A thin-cap fibroatheroma, or TCFA, is a plaque with a large lipid core covered by a dangerously thin fibrous cap. Intravascular OCT can detect these structures and measure the cap thickness, which gives interventional cardiologists information about which lesions are most likely to cause trouble. Automated methods for detecting lipid plaques and measuring cap thickness are being developed to speed up this analysis during procedures.
That said, the technology is not perfect for this task. One study comparing intravascular OCT readings to actual tissue histology found that only about 38% of plaques called TCFA by OCT were true TCFAs on biopsy. Foam cell infiltration was responsible for the majority of false positives, making thick-capped plaques look thin or creating the appearance of a lipid core where none existed. When the researchers relaxed one of the standard diagnostic criteria, sensitivity for detecting genuine TCFAs climbed to 87% while specificity remained at 92%, suggesting the traditional imaging criteria may be too strict in some respects and too loose in others.
Beyond plaque characterization, intravascular OCT has become valuable for optimizing stent placement. After a stent is deployed in a narrowed artery, OCT can show whether the stent is fully expanded, whether its struts are sitting flush against the vessel wall, and whether there are gaps or areas of incomplete contact. In calcified lesions, where stent malapposition is more common, this kind of detailed feedback helps operators decide whether to post-dilate or adjust their technique.
Skin Cancer Detection
Dermatology is a natural fit for OCT because the structures of interest, the epidermis and superficial dermis, sit within the technology’s imaging depth. The most studied application is the detection and subtyping of basal cell carcinoma, the most common skin cancer. A Cochrane systematic review pooling data from studies that used conventional swept-source OCT alongside visual inspection and dermoscopy estimated a sensitivity of 95% and specificity of 77% for detecting basal cell carcinoma. Applied to a hypothetical group of 1,000 suspicious lesions with a 60% prevalence of basal cell carcinoma, OCT would miss about 31 cancers, which is 91 fewer misses than visual inspection alone and 53 fewer than visual inspection combined with dermoscopy. It would also produce about 93 false positives, a meaningful reduction in unnecessary excisions compared to the alternatives.
A prospective cohort study found that adding OCT to standard clinical examination improved diagnostic accuracy for basal cell carcinoma and helped distinguish superficial subtypes from deeper ones. The distinction matters because superficial basal cell carcinoma can often be treated with topical therapies or superficial destruction rather than surgical excision, so getting the subtype right can spare a patient from a more invasive procedure. Combining OCT with reflectance confocal microscopy, which images at a different scale, has shown even higher sensitivity in some settings, reaching 100% in one study of previously unbiopsied suspicious lesions, though with lower specificity.
Barrett’s Esophagus and the GI Tract
OCT has also found a role inside the gastrointestinal tract, delivered through endoscopic probes. One of the most promising applications is in Barrett’s esophagus, a precancerous condition in which the lining of the lower esophagus changes in response to chronic acid reflux. The standard approach to surveillance involves taking random biopsies, which can miss patches of early dysplasia because the abnormal tissue is not always visible to the naked eye. OCT-based angiography features, specifically abnormal vessel branching and uneven vessel sizes in the esophageal lining, have shown 94% sensitivity and 69% specificity for distinguishing dysplastic tissue from non-dysplastic Barrett’s, with an average reading time of just 45 seconds per dataset. A systematic review and meta-analysis concluded that OCT-guided targeted biopsies could improve detection of dysplasia and early neoplasia compared to the standard random-biopsy protocol.
Visible-Light OCT and Oxygen Measurement
Most clinical OCT systems operate in the near-infrared range, around 800 to 1,300 nanometers, because those wavelengths penetrate tissue reasonably well and are invisible to the patient. But a newer approach uses visible light, typically in the 500 to 600 nanometer range, which trades some penetration depth for significantly higher axial resolution and a unique biological trick: the ability to measure blood oxygen saturation. Hemoglobin absorbs visible light differently depending on whether it is carrying oxygen, and visible-light OCT can exploit those differences to perform retinal oximetry without any dye or additional equipment. This is an emerging method, and pilot studies have begun exploring whether oxygen measurements in the macula could serve as a biomarker in conditions like glaucoma, where impaired blood flow may contribute to nerve damage.
Using longer wavelengths pushes in the opposite direction. Researchers comparing OCT at 1,300 nanometers versus 1,600 nanometers found that the longer wavelength provided up to 30% greater imaging depth in highly scattering samples because scattering drops off at longer wavelengths. The practical limit is water absorption, which increases at longer wavelengths, so the benefit is greatest in tissues with low water content. These wavelength trade-offs are one reason different OCT systems are optimized for different body sites.
Specialized Extensions of OCT
Beyond standard structural imaging, engineers have developed several specialized OCT variants that extract additional types of information from tissue.
Polarization-sensitive OCT measures how tissue changes the polarization state of light passing through it. Collagen-rich structures like tendons, cartilage, and scar tissue are birefringent, meaning they bend polarized light in a way that depends on fiber orientation and density. This makes polarization-sensitive OCT useful for assessing cartilage integrity in osteoarthritis or evaluating scar tissue after a heart attack, where the collagen content and organization of healing tissue carry prognostic information.
Optical coherence elastography uses OCT to measure tissue stiffness. A small mechanical stimulus, such as a puff of air or a vibration, is applied to the tissue, and OCT tracks the resulting deformation at micrometer-scale resolution. Because many diseases change tissue stiffness, from fibrosis stiffening the liver to tumors hardening breast tissue, elastography has the potential to add a functional dimension to what has traditionally been a purely structural imaging method. The field has been active since the late 1990s and continues to develop, though clinical adoption remains limited compared to the structural and angiographic forms of OCT.
OCT in the Operating Room
Integrating OCT directly into surgical microscopes has given ophthalmic surgeons a new way to see what they are doing in real time. Microscope-integrated OCT systems overlay cross-sectional or volumetric images onto the surgeon’s view, showing the position of instruments relative to tissue surfaces that are otherwise difficult to judge by visual inspection alone. In retinal surgery, for instance, peeling a membrane off the retinal surface requires gauging the separation between the membrane and the underlying tissue at a scale of tens of micrometers. The surgeon’s stereoscopic view through the microscope gives some depth cues, but OCT provides a quantitative cross-section that removes much of the guesswork.
Prototype systems have demonstrated real-time volumetric imaging during both anterior segment and retinal surgeries, with stereoscopic data visualization delivered through a heads-up display. Results from dozens of human surgeries have shown that surgeons can see flap initiation, flap removal, and retinal deformation during instrumentation in near real time, information that was previously invisible during the procedure.
Beyond Medicine
OCT’s combination of micrometer-scale resolution, non-contact operation, and speed has attracted attention from industries that need to inspect materials without destroying them. The technology has been explored for structural analysis of composite materials, quality inspection of printed electronic devices, monitoring thin-film coatings, examining printed circuit boards, and analyzing industrial fluids. In display manufacturing, for example, OCT can measure the thickness of individual layers in a multi-layer stack and detect delamination or voids without touching the product. In laser-based manufacturing, it can monitor weld penetration depth in real time.
The industrial appeal is straightforward: OCT does what destructive cross-sectioning does, but without destroying the part, and it does it fast enough to keep up with a production line. As manufacturing tolerances shrink and the demand for in-line quality control grows, OCT’s role outside medicine is expanding.
Miniaturization and the Endoscopic Frontier
One of the persistent engineering challenges has been making OCT probes small enough to fit inside the body’s narrowest spaces. Micro-electro-mechanical systems, or MEMS, have been central to this effort. MEMS-based scanning mirrors can steer the OCT beam across tissue while fitting inside a probe just a few millimeters in diameter. One prototype forward-viewing endoscopic probe measured 5.5 millimeters across and 55 millimeters long, with the scanning mirror operating at less than two volts. Another design enclosed a two-axis MEMS mirror and a silicon optical bench inside a waterproof polycarbonate tube with a built-in lens, designed for in vivo use.
These probes matter because many of OCT’s most promising applications, from imaging coronary arteries to scanning the esophagus or biliary ducts, require getting the optics deep inside the body through a catheter or an endoscope channel. The smaller and more flexible the probe, the more anatomy becomes accessible. Ongoing miniaturization is one of the factors most likely to determine where OCT shows up next in clinical practice.